The present invention relates to the field of the resonant radiofrequency ignition of an internal combustion engine. It relates more particularly to a device designed to measure the ionization current of the gases in the cylinders of the engine.
The ionization current of the gases in the cylinders of the engine is typically measured after the end of ignition and is then used to perform diagnostics on the progress of the combustion, for example in order to detect the angle corresponding to the maximum pressure of the combustion chamber, to detect pinking or even to identify combustion misfires.
Circuits for measuring the ionization current for a conventional ignition system are known, the operation of which consists in polarizing the air/fuel mixture present in the combustion chamber after the generation of the spark between the electrodes of the spark plug, in order to measure the current resulting from the propagation of the flame.
These circuits however have to be dedicated to the characteristics of conventional ignition and are not adaptable as such to the plasma generation ignition systems that use spark plugs of radiofrequency plug coil (BME) type, as described in detail in the following patent applications filed in the name of the applicant: FR 03-10766, FR 03-10767 and FR 03-10768.
As it happens, the specifics of radiofrequency ignition cause a number of constraints for measuring the current deriving from the combustion.
First of all, the ignition command signal induces significant currents which have an amplitude difference of more than 120 dB with the ionization current due to the combustion of the combustible mixture. Since this current is measured after the end of ignition, there is therefore a glare time, during which the measurement circuit cannot acquire a weak current.
Furthermore, since the measurement circuit is inserted into the ignition system, it is important not to significantly reduce the efficiency of the ignition system.
Finally, this type of radiofrequency ignition makes it possible to develop two types of discharges, a multi-filament spark and a mono-filament arc, which influence the ignition system differently. There is therefore a difficulty in guaranteeing independence of the measurement of the ionization current with respect to the type of discharge generated.